in valve trays, perforations are covered by liftable caps. Vapour flows lifts the caps, thus self creating a flow area for the passage of vapour. The lifting cap directs the vapour to flow horizontally into the liquid, thus providing better mixing than is possible in sieve trays.

in klepbakke word perforasies met opligbare kleppe bedek. Die kleppe word deur dampvloei gelig, wat dus vanself 'n vloeiarea skep, waar dampe kan deurgaan. Die opligbare klep rig die damp om die vloeistof horisontaal binne te vloei, wat dus beter vermenging, as wat met sifbakke die geval is, verseker.